The advice comes from neighbors, magazine articles, clinic websites, even nurses: drink a lot of water before and after your COVID-19 vaccine to help ward off side-effects.
The problem: There is no evidence that drinking extra water can help ward off the sore arms, body aches, and fevers that some people experience after getting their COVID-19 vaccine shots.
Water-chugging also won’t reduce the chances of fainting for people who are prone to lightheadedness with needles.
Here’s what we know about how water might influence your vaccine response and general health, based on the evidence available.
Vaccination and water
Scientists have not conducted randomized trials to see how drinking—or not drinking—water before getting your injection might affect antibody levels or other immune responses. It’s a complicated question to sort out, in part because the immune response follows two main paths: In the long-term, it helps the body mount lasting defenses against the virus. On a shorter time-scale, the vaccine also causes the “innate” immune response, which is responsible for side effects some people feel after they get their shots. Researchers have conflicting opinions about water’s role in all of it.
Studies in frogs (a distant relative to humans) suggest that extreme dehydration could suppress the immune system, making it harder for cells to signal each other, says Sonia Sharma, an immunologist at the La Jolla Institute for Immunology, in California. In people, dehydration could be one of multiple stressors and poor health behaviors that delays the production of antibodies, she adds. And some research suggests that people feel more pain when they are dehydrated, says Jodi Stookey, a nutrition epidemiologist formerly at Children’s Hospital Oakland Research Institute, in California.
But drinking too much is also a health risk, studies show, causing sodium levels to drop and leading to headaches, fatigue, seizures, even death. And plenty of experts argue that, outside the context of excessive heat or endurance exercise, healthy adults easily get enough liquids through food and beverages—even if they feel under the weather for a day or two after a vaccine shot.
And while water might play a role in preventing kidney stones and recurring urinary tract infections, for people trying to maximize their immune response while minimizing side effects, water is unlikely to do the trick on its own. “Water is not the magic bullet that’s going to give you this optimal immune response,” Sharma says. “It’s part of the cache of healthy behaviors that promote a healthy immune system.”
Research with endurance athletes raises doubts about water’s potential to influence the immune system at all. Prolonged exercise, such as marathon running, is known to cause a rise in stress hormones that can reduce white blood cell function for several hours and make runners more susceptible to getting sick right after a long effort, says Michael Gleeson, an emeritus professor of exercise biochemistry at Loughborough University, in the United Kingdom, who studies nutrition and immune responses to exercise.
White blood cells include the T cells and B cells that find infectious agents, formulate a defense, and develop antibodies that recognize and remember them. When researchers assigned runners to drink more and maintain hydration during their runs, however, their immune systems showed the same level of suppression compared with runners who got dehydrated.
“The notion that drinking lots of water can help you avoid side effects of the COVID vaccine sounds ridiculous,” says Gleeson. “Water does not influence immune function.”
Water-drinking is also unlikely to help with fainting. In about one out of every thousand vaccine doses given, shots trigger a vasovagal reaction that causes dizziness, lightheadedness, and sometimes passing out within the first 15 minutes. Based on evidence that people are less likely to faint if they drink water before donating blood, Alex Kemper, division chief of primary care pediatrics at Nationwide Children’s Hospital, in Columbus, Ohio, assigned hundreds of people between the ages of 11 and 21 either to drink up to two cups of water in the hour before their shot or to act as usual. Extra drinking, he found, didn’t change their likelihood of getting dizzy or feeling like they were going to pass out. “The bottom line was, it didn't make a difference,” Kemper says.
Questions about hydration and vaccine response fit into a bigger set of questions about how much water we need, how to measure dehydration, and whether we need to be intentional about drinking in general. Older adults are most vulnerable to chronic dehydration, data suggest, but how common it is remains unclear.
How to assess your own hydration status is also up for debate. Thirst may be enough, according to Kemper. “Millions of years of evolution has made it so that when we really need it, we’ll drink,” he says. “On a typical day, you can let your innate drive to drink when you’re thirsty do it for you. And probably the same is true on vaccine day.”
Given the uncertainties, the advice to drink water may tap into people’s desire for a sense of control. It is simple and easy to do, and some experts see the pre-vaccination hydration advice as harmless and possibly even beneficial if it helps motivate people to get their shots. “The more people that we can get vaccinated, the better, and if people want to drink water, it’s certainly not going to hurt,” Kemper says. “People put a lot of faith in water, and if it makes people feel better, what’s the harm?”
But others worry that, if water doesn’t help, encouraging what might essentially be a placebo could cultivate distrust in the medical system and suggest that normal side effects are something to worry about. Indulging in unscientific health fads can harm the credibility of health care providers, adds Christopher Labos, a cardiologist and epidemiologist at Queen Elizabeth Health Complex, in Montreal.
In his opinion, telling people to drink water does them more of a disservice than saying, “Look, it’s a sore arm. It’s going to pass, so just ride it out,” says Labos, who has written about water myths. “While it’s harder in the short term, it probably does more good in the long term to just be honest with people and tell them that this is not a significant health issue.”
In his practice, Labos says, he fields a lot of questions from patients about what they should consume or avoid when they get the vaccine. “What you eat, what you drink, the medications you take are not going to affect the vaccine,” he says. “When it’s your turn to get vaccinated, just go get vaccinated.”